CWCT CURTAIN WALL INSTALLATION HANDBOOK

This handbook was written by the Centre for Window and Cladding Technology (CWCT) aspart of its training programme to improve the standard of curtain wall installation. It will be of benefit to all those installing, or supervising, the installation of curtain walling andother glazed building elements. This handbook was part-funded by the Department of the Environment, Transport and theRegions under research contract number 39/03/272 cc 862. The CWCT is sponsored by:

Ove Arup & PartnersBovis Lend Lease LtdComar Architectural Aluminium Systems LtdCouncil for Aluminium in BuildingKawneer UK LtdPilkingtonTechnalTaywood EngineeringUniversity of Bath

All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any meansincluding photocopying and recording without the written permission of the copyright holder, application for whichshould be addressed to the publisher. Such written permission must be obtained before any part of thispublication is stored in a retrieval system of any nature. Centre for Window and Cladding Technology November 2001 ISBN 1 874003 96 3 Published by Centre for Window and Cladding Technology, University of Bath, Claverton Down, Bath BA2 7AY

Contents

Chapter Page

1 The façade 6

2 Principles of weathertightness 11

3 Frames 19

4 Gaskets 27

5 Sealants 32

6 Finishes 41

7 Glass 45

8 Brackets and fixings 56

Introduction The installation of facades and façade elements is one of the more complex site operations.It requires a range of skills and knowledge yet has not been recognised as a particular skillor trade. Façade failure, particularly water leakage, is the most common cause of failure innew buildings. This handbook brings together advice on installation of curtain walling including all the majorcomponents: frames, gaskets, sealants, finishes, glass and fixings. It is based onexperience gained by CWCT in setting up training centres for installers and in training maincontractors’ site supervisors. The book explains why things should be done and highlights those things that are mostcritical to the success of curtain wall and window installation. This handbook is a guide to achieving better curtain wall installation. However, it is not asubstitute for care and diligence, nor should it be a substitute for proper training. Full detailsof CWCT’s training programme are available at http://www.cwct.co.uk/installers.

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1 The facade

� FunctionThe facade of a building has to exclude the weather, provide a comfortable internalenvironment, be safe during construction and use, and retain its appearance throughout itslife. Facades will only do these things successfully if they are correctly designed, planned andinstalled. This guide gives advice on the correct installation of facades and the componentsthat make up a facade. Modern facades are often highly technical involving the use of many materials, Figure 1.1.

� ComponentsFacades are made up of components or elements. These are factory-made to hightolerances and quality. However each is designed as a separate self-contained componentwithout full regard as to how it may be built into most forms of facade construction. It is leftto the facade designer and the installer to detail and fit the component on any particularcontract. The different elements of the facade are each selected to serve a purpose andmay not be chosen for ease of installation.

- WindowsWindow types are selected to provide ventilation, ease of cleaning, ease of operation,appearance, escape in case of fire, resistance to burglary and blast. - DoorsDoor types are selected for security, appearance, emergency exits, fire performance, easeof operation, robustness.

- GlassThere are many types of glass to meet the requirements of particular buildings. Glass canprovide security, resistance to blast loading, safe failure, strength, reduced soundtransmission, reduced glare, reduced transmission of ultra violet, colouration andappearance. - PanelsInfill panels in glazing frames and panels mounted as rainscreen are selected for theirappearance, strength, resistance to abrasion and vandalism, fire rating, strength, ease ofinstallation. � TypesFacades take many forms ranging from heavy forms of construction; brickwork and precastconcrete to the lighter forms such as profiled metal sheet, stick curtain walling and glassscreens. The form of construction will have been chosen by the client and the architecthaving full regard for the purpose of the building, the image required, the design life of thebuilding and whole life costing (energy and maintenance costs).

The basic forms of cladding are:

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- Brick and blockworkLoad bearing masonry is the traditional method of building low-rise buildings without astructural frame. Masonry in medium and high rise construction is normally built as a non-load-bearing wall supported by the structural frame at floor levels. Windows and doors arefitted to holes left as the masonry walls are built. Masonry walls normally have cavities and itis important that factory made components are correctly sealed into the multi-layer masonrywall. The accuracy of construction is at odds with the accuracy required at joints. - In-situ and precast concreteIn-situ concrete may be used to form the exposed surface of a façade but is more commonlyconcealed by an external cladding or rainscreen. Precast concrete is normally non-loadbearing and may be used to form cladding panels or backing wall panels clad withrainscreen.

Windows and doors may be installed into openings in in-situ concrete walls or precastpanels, either at site or, for precast panels, in the factory. In other cases window openingsmay be formed as spaces between precast units. Components must be correctly sealed intothe panels. If windows abut more than one panel then special care is needed to fix bothpanels and windows to prevent unintentional movement. - Panelized curtain wallCurtain wall may be constructed as large panels. Each the width of a structural bay and onestorey high, they can weigh up to 15 tonnes, Figure 1.2. They may be precast concretepanels or steel trusses to which are attached outer and inner surfaces, insulation, andwindows. Components are fitted onto the panels in much the same way that they are fittedinto other facades. Panels of this size require very large fixings and anchors to hold them onthe building. Special attention should be given to the large panel-to-panel seals that arerequired. - Unitised curtain wallSmaller factory-made panels are used in unitised construction. Typically one glazing bay inwidth and one or two storeys high the units are either:- stick curtain walling frames that are factory assembled as ladders- panels of concrete, gfrp, grc, metal skinned insulating composites that are factoryassembled and include windows as required

Sealing the joints between units on site often depends on good workmanship andunderstanding of joint behaviour. - Stick system curtain wallCurtain wall can be formed from a stick system of site assembled framing members,mullions (vertical) and transoms (horizontal). Glazing and infill panels are fixed into theframing grid by clamping them in to a glazing rebate Figure 1.3. Panels may also be fittedas rainscreen, structural silicone glazing or bolted structural glazing.

Stick curtain walls are usually built from standard systems but they always have non-standard interfaces with adjacent elements (roof, structure, other wall elements). Stickcurtain walls can be custom-designed to include accessories such as blinds and brise soleil.

- RainscreenRainscreen is constructed as panels with a ventilated cavity between them and an inner airbarrier. Rainscreen is either built by mounting support rails and panels on an inner wall ofconcrete, brick or blockwork (overcladding) or is part of a curtain wall (panelised, unitised orstick) that is self-supporting with integral cavity and air barrier (integral rainscreen) Figure1.4. The panels may be of any material including metal, gfrp, stone, glass and ceramics.

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- Bolted glass assembliesGlass is either bolted directly to a supporting frame or a number of pieces of glass are boltedtogether to form a structural glass assembly. The installation of these walls may requiregreater knowledge and skills than are described in this book.

- Profiled metal claddingProfiled metal normally spans between sheeting rails or purlins supported by the structuralframe. It may be used in one of two basic forms: single skin and double skin insulated. Thesecond form is used for cladding heated habitable buildings. The fitting of windows anddoors requires attention to air and water sealing of joints with complex shapes.

� DurabilityAll facade components will deteriorate and age. This results from weathering, abrasion,staining, mechanical wear and tear. The useful life of a wall and the period to first repairmay be reduced if any component is incorrectly installed or substituted with an inferiorproduct. Walls are generally required to last in excess of 20 years. A normal requirement would befor the primary framing members to last 40 or 60 years while other components such ashardware are required to last 20 years before refurbishment or replacement. Othercomponents such as sealant joints may be designed to have a shorter life.

Poor installation can reduce the useful life of components to less than half of that intended.In particular inconsistent workmanship can lead to premature failure of a few componentsacross the whole facade or complete failure of a small area of wall. With medium- and high-rise buildings the cost and difficulty of gaining access for remedial work will be far greaterthan any savings made by using inferior materials or modifying the design at site to simplifyinstallation.

� InterfacesA wide variety of components and wall elements are brought together in differentcombinations on every building site to create a unique building. Components are designedby manufacturers to fit into a number of construction forms but the interface betweendifferent manufacturers' components and constructed elements such as brickwork is theresponsibility of the specialist contractor. Particular problems arise when the work of twocontractors meet at an interface and design responsibility is shared.

Late substitution of one component for another, such as a window, will often require thedesign of an interface to be changed. This guide deals with the principles and practice ofinstallation to ensure that interfaces are properly detailed and constructed at site level.

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Cost Materials Performance Quality Appearance

- Capital cost….

- Running costs….

- Whole life costsEnergyMaintenance

---

- GlassAnnealedToughenedLaminated….

- MetalsAluminiumBronze….

- Plastics….

- Stone

- SealantsSiliconePolysulfideAcrylic….

- Gaskets….

- Finishes….

-

- WeathertightnessWaterAir….

- Wind loading….

- ThermalU-valueSolar gain….

- Condensation….

- Ventilation….

- Acoustics….

- FireResistanceReaction….

--

- Methods….

- Standards….

- Inspection….

- Testing

- Fit….

- FinishesGlossColour….

- ShapeFlatnessCurvature

---

Figure 1.1 Some of the aspects of wall performance to be taken into account during design andinstallation

Figure 1.2 Unitised/panelised curtain wall

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Figure 1.3 Stick system curtain wall

Figure 1.4 Rainscreen panels

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2 Principles of weathertightness

� Water and air tightnessIt is important that a facade keeps out the rain and the wind. Walls are designed to resistwind loading appropriate to the site and to provide water penetration resistancecorresponding to that wind exposure.

Walls are designed to achieve the required low levels of air leakage. The allowable leakageis determined by the specifier and will depend on the use for which the building is designed,whether or not it is air-conditioned and the assumptions made when designing the heatingsystems. Excess air leakage gives rise to increased heating costs and possibly an inabilityto heat the building fully.

� Water penetrationWater should not penetrate the wall and reach the inner surface of the wall. It is alsounacceptable for water to penetrate partly through the wall if it causes damage (rot orcorrosion) to the wall or reduces its performance (reduction of thermal insulation).

A wall may be designed and constructed so that water can enter into the wall but is thendrained safely to the outside. Water management rather than watertightness is the secret toconstructing a good wall.

Water will penetrate the wall wherever there is an opening, water and a mechanism to takethe water through the opening.

� How water penetrates a wallWater may penetrate a wall or component in one of six basic ways:

- Gravity- Wind pressure- Air borne- Kinetic energy- Surface tension- Capillary action

These are illustrated in Figure 2.1. Very often it is a combination of factors that causesleakage.

Incorrect installation can allow water to enter by any of these mechanisms even if the wall isdesigned to prevent water penetration.

Failure to lap components such as flashings, wrongly fitted gaskets and poor sealant jointswill all create openings that allow water to flow into the wall under gravity. If drainage pathsare blocked water will pond and overflow (often into the wall) under the effect of gravity.

Failure to seal openings that should be sealed and the incorrect fitting of gaskets leavesopenings through which the wind can force water.

Failure to install air seals correctly allows air to pass through the wall and this may carrywater into the wall.

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Removal of drips and nibs from the underside of components can allow water to remainattached to the surfaces and run into the wall as a result of surface tension.

� Leakage pointsGravity is the most serious cause of water leakage followed by the effects of wind pressure.Both can allow large volumes of water to flow continuously. The other causes of leakageallow only intermittent flow or small flows.

The risk of leakage is greater at places on the wall where there is most water or greatestpressure. The points at which water leakage is mostly likely to occur are shown in Figure2.2. Water is driven across the facade by the wind. It gathers at the mullions and runs downto the corner of each frame bay.

Wind passing a building moves around and over the building. This movement of the winddeposits more intense rain on the edges of the facade. Wind moving upwards on the wallcan drive rain up the wall, particularly on the upper levels of medium and high rise buildings.Drainage openings can be designed to cope with this. Water may leak past gaskets andseals at the head of a frame if the joints are not correctly made.

It should never be assumed that a joint is in a protected position and that it is not animportant joint.

The use of picture frame gaskets avoids the need to make mitre or butt joints of gaskets onsite.

� Forms of constructionWalls have to be sealed against air leakage and water has to be prevented from penetratingthe wall. In many walls and components the air seal is separate from the water barrier.

The outer water barrier, or rainscreen, prevents large amounts of water entering the wall orcomponent. A cavity in the wall or component frame intercepts the small amount of waterthat passes the water barrier. An inner air barrier or air seal is provided behind the cavity togive the required low level of air leakage.

The water barrier is designed to prevent water leakage. It is the primary defence againstwater leakage and should be constructed with this in mind. Any small amounts of water thatenter the cavity have to be drained to the outer face of the wall.

The outer layer may be impermeable such as an aluminium or glass panel or it may beporous such as brick or terracotta.

� Drained facadesWith the exception of front sealed construction, all framing members and cavities behindrainscreen panels should be designed to be drained. This means that water passing theouter seal has to drain out through drainage openings to the outer face.

Drainage may occur at open joints between panels or through drainage paths in the framingmembers.

Window frames are normally drained through holes in the outer face of the frame. Anopening sash may have drainage holes in its lower edge. These drain water from the

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glazing cavity into the cavity below. This in turn has to be drained to the outer face, Figure2.3.

Stick curtain walling systems may be drained in the same way as windows. Each glazingrebate is drained to the outer face with holes in the front face of the lower framing member,Figure 2.4. Alternatively systems may be designed to drain water along the transom to themullions, Figure 2.5. The drainage capacity of these systems is limited and water should bedrained from the mullion at every third floor.

It is important that drainage channels are not blocked as the wall is installed. Badly placedglazing blocks, use of sealants in the wrong place, debris left in the glazing rebate orinadequate or missing drainage holes can all block the intended drainage paths.

Water will not drain freely from very small openings due to the effect of surface tension.Drainage holes should be at least 8mm diameter or 25mm x 6mm. Holes that are partiallyblocked or not properly deburred will not allow water to drain. Glazing blocks should bridgethe drainage channel in the glazing rebate unless drainage holes are provided between allglazing blocks. Water will not drain for long distances along horizontal frames, particularly ifthey deflect under load. Many designs set a maximum distance between drainage holes.

� Drained and ventilated facadesDrained rainscreen and glazing frames have drainage holes at the bottom of each cavity toallow water out. Holes may also be provided at the top of the cavity to provide ventilation ofthe cavity. This allows air to pass through the cavity or frame to remove excess watervapour.

Holes for ventilation may be smaller than drainage holes. They are normally made the samesize as drainage holes and placed in symmetrical positions so that transoms cannot beinstalled upside down. If transoms have holes for only one glazing rebate it should beassumed that they are drainage holes. The transom should be placed with the drainageholes uppermost so that they are at the lowest point of the glazing rebate they drain.

� Pressure equalised facadesPressure equalised windows and walls are designed with openings large enough to allow theair pressure in the cavities to nearly balance that of the wind on the outside. This helps toprevent water from entering the cavity.

For a rainscreen the drainage and ventilation holes may be larger than for a simply drainedand ventilated system. Pressure equalised window frames do not always need larger holes.It is not obvious on site whether a window is pressure equalised or only drained andventilated.

Unless a seal is shown it should be assumed that all holes, including ventilation holes, arenecessary to prevent water penetration into the wall.

If a glazing rebate is vented into the cavity between an opening frame and a fixed frame thenthe outermost vent or drain holes will be larger than the inner ones. This is because theyhave to allow pressurisation of the two cavities, Figure 2.6.

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� WindowsWindows are tested to BS5368 Pt 1 for watertightness and BS5368 Pt 2 for airtightness.The test only proves the effectiveness of the window and its internal seals. The jointbetween the window and the surrounding wall is equally important if the wall is to functioncorrectly.

The window should be sealed to the surrounding wall using either a wet applied sealant, asealant impregnated sponge or, in the case of a window in a curtain wall, a gasket. Thesame principles apply to these seals as to all the other seals of the window:

- The seals should be well made.- It should always be assumed that some water may leak past the outer seal and

provision should be made to drain this to the outer face.- An effective air seal should be made at the inner face, Figure 2.7.- The sill should be sealed to both the surrounding wall and the window taking care not

to restrict any drainage channels.

Particular attention should be paid to the sill detail. Sub sills are not tested as part ofBS5368 and in any case are frequently made to suit a particular contract.

� Curtain wallCurtain wall is tested to the CWCT ‘Standard and Guide to Good Practice for CurtainWalling’. A representative sample of the wall is tested and for a custom wall the test willhave included the flashings and typical interface with adjacent elements of the buildingenvelope.

Flashings and interfaces must be constructed in accordance with the drawings as approvedafter test.

For proprietary systems it is normal to test the system once only. In this case the flashingsand other interface details may not have been tested. It is important that these details arebuilt according to the drawings.

The installer should be alert to any possible leakage paths as this is often the first time thatcomplex details have been seen full size in three dimensions. If there is doubt about thedetailing of the interfaces the designer should be consulted before work continues.

If a wall has been tested the design may have been modified as a result of early tests.

The wall should be constructed on site to exactly match the wall tested. The installer shouldbe notified of any modifications or non-standard details. Even proprietary systems aremodified from contract to contract and the installer should not assume familiarity with asystem.

� RainscreenRainscreen performance depends on the rainscreen panels and all components in thecavity. Design drawings and test reports should show details of the framing members,number and location of fixings, size and position of all openings, dimensions of all cavities,internal flashings and gutters, cavity closers and fire barriers.

Excess water may pass the rainscreen if:- drainage and ventilation openings are the wrong size

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- cavity closers are omitted or wrongly constructed- baffles are omitted from joints- the cavity is too wide

Water will fail to drain from the cavity if: - drainage holes are too small - drainage paths are blocked with debris - the cavity is blocked with insulation material - internal flashings and gutters are incorrectly fitted or missing - drainage of components (windows and doors) is not linked with drainage of the rainscreen

� Site testingSite testing may be carried out during construction to check for good workmanship andconsistent performance. Testing may also be carried out after construction to identify thecause of water leakage.

The hose pipe test is used for routine site testing for water penetration. The test is describedin CWCT ‘Standard and guide to good practice for curtain walls’. A full description of sitetesting is given in CWCT TN10 ‘Site testing for watertightness’. Testing should beconducted using a standard nozzle, standard water pressure and motion of the nozzle.

The test was developed for use on sealed joints but it may be modified for use on openingjoints. In this case it is good practice to vary the nozzle pressure and not the motion of thenozzle.

� Air leakageAir leakage can lead to excessive heating bills, an inability to heat a building anduncomfortable draughts

Allowable air leakage rates are given in Part L of the Building Regulations.

High rates of air leakage are symptomatic of poor installation. Walls that leak too much airare also likely to leak water as poor air seals impair the pressure equalisation of many wallsand components.

High rates of air leakage are associated with unintentional openings in the air barrier. Theseopenings will impair the acoustic properties of the wall and allow more sound into thebuilding.

The main causes of unintentional air leakage are:- incorrectly fitted air seal gaskets- failure to seal windows and other elements to the air barrier of the surrounding wall- Opening windows and doors that are not correctly adjusted and do not seat correctly

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Figure 2.1 a) Gravityb) Kineticc) Surface

tension

d) Capillary actione) Air drivenf) Pressure

difference

Figure 2.2 Potential leakage sites

Figure 2.3 Drainage of window frames

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Figure 2.4 Transom drained curtain wall

Figure 2.5 Mullion drained curtain wall

Figure 2.6 Pressure equalised frame

18

Figure 2.7 Air sealing of window to wall

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3 Frames

� Function Frames may be used for windows, glazing screens and curtain walls. In all cases the frameis composed of a series of profiles assembled to form the frame and designed to supportglazing or other infill panels. For windows, assembly of the frame, and sometimes glazing, iscarried out before delivery to site whereas for glazing frames and curtain walls at least some,and in some cases all, of the assembly work will be carried out on site. � Frame materials Framing materials are selected largely on the basis of individual or corporate preference.They are chosen because of the specifier’s familiarity with the material or for reasons suchas ‘green issues’. Each material offers different benefits and this may determine the choiceof material. The principal materials used to form glazing frames are: - Timber Traditionally used as a framing material, today both hardwood and softwood are used.Timber suffers from rot but modern timber treatments combined with good design andworkmanship give an acceptable life. However regular maintenance of finishes is required.Timber windows may be produced with a drained glazing cavity but many are undrained andrely on a single outer seal between glass and frame to keep out the water. Water ingressfollowing failure of the seal can then lead to failure of the edge seal of double glazing units.Many windows have a limited depth of rebate restricting the width of glazing unit that can beaccommodated. Timber has been used for glazing screens but this is not common.

Timber is used as a solid section and is thus relatively stiff. It resists bending and torsionwell and hardware can be attached to the frame with little difficulty.

- SteelSteel was introduced as an alternative to timber for window frames. Originally hot rolledsections were used but today steel is used as cold-formed sections to make window anddoor frames. Steel windows are galvanised and powder coated and may today be double-glazed. The hardware is usually an integral part of the window. Steel windows allow the useof slender sections yet are robust and are comparatively secure, if secure hardware is used.Steel has obvious advantages when making fire resistant glazing screens and windows.

- AluminiumAluminium has been used as a framing material for some fifty years. Aluminium does notsuffer badly from corrosion and is easy to form and finish allowing many different designs.Aluminium is used as hollow sections and is relatively flexible in bending and torsion.Hardware often has to be matched against a particular profile. Because of the hollow andcomplex profile cross sections achievable with aluminium it is easy to make drained andventilated or pressure equalised windows. Aluminium is a very good conductor of heat. To meet requirements for low thermaltransmission aluminium profiles are thermally broken with either a polyamide or resinelement between inner and outer aluminium sections. The latest proposals for improving thethermal performance of windows will require improved thermal breaks. Aluminium is the most common frame material for stick system curtain walls, glazing screensand shop fronts. It is also commonly used as a framing system to support rainscreens.

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- PVCuPVCu is a flexible material that is normally internally reinforced with steel or aluminium togive it the required strength and stiffness. As with aluminium it is easily formed to produce awide variety of profiles. When first introduced it was generally used white without anyfinishes but it is now widely available in coloured form, either using coloured material, foilfinishes or specialist paints. It is dependent on reinforcement for its strength and hardwareshould be fastened through to the reinforcement. Many PVCu profiles are multi-chamberedand it is essential that they drain correctly.

PVCu is now used to construct glazing screens and low rise curtain walling. The structuralelements are PVCu clad aluminium and these form the supporting grid for PVCu framedwindows.

- CompositesThe use of composite frames allows the designer to use the advantages of differentmaterials for the inner and outer parts of the frame. Common combinations are:

Aluminium - PVCu Aluminium - Timber Stainless Steel - Aluminium Bronze - Aluminium Composite frames are used to improve thermal performance (heat loss), reduce the risk ofcondensation, give a more durable outer weathering surface, give different appearances tothe inner and outer finishes.

� Window typesThere are many different types of window in use. Some of these are traditional designs,others are copied from traditional designs elsewhere and some have only become possiblewith the use of modern materials and hardware.

The window types commonly used in the UK are:

Fixed lightSide hung ventProjecting sidehungTop hung ventProjecting top hungTilt-turn

Vertical sliderHorizontal sliderHorizontal pivotVertical pivotOff-set vertical pivot

These are shown in Figure 3.1. The drawing notation used is in accordance with BS4873 inwhich the arrow drawn on the glazing points toward the hinges.

UK practice has been to use a solid line for open out windows and a dotted line for open inwardwindows. This is different from practice in some European countries and it should be clearlyestablished which convention is being used.

� Window selection The types of frame used on any particular contract will depend on a number of factors.These include:

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- MaintenanceWindows that can be cleaned from the inside of the building may be preferred where it ispossible to use a large proportion of opening windows. Framing materials that require littlemaintenance are also preferred.

- Safety in useWindows have to be safe in use. They may have to meet any of these needs:- be safe to clean and maintain- provide a fire escape route- prevent people from falling out - not obstruct paths and passages when open - VentilationWindows of different types give different ventilation patterns in a building Figure 3.2. Thesize of the opening sash will determine its weight and the hardware to be used. - Local customWindows will often be selected to match those on nearby buildings. For refurbishment theyare normally chosen to follow the style of earlier windows. On listed buildings and inconservation areas it may be a requirement that particular windows are used.

- Size of openingThe size of window opening will depend among other things on the lighting requirements,view, allowable heat loss and appearance of the window.

- Preferred materialFraming materials may be selected on the basis of cost, durability, strength, appearance.Increasingly whole life costs and environmental issues are being taken into account.

- Glazing materialThe glazing or infill material may affect the choice of framing material. The frame has tosupport the weight of the glazing and accept glazing units of the required thickness. - AppearanceThis probably has the greatest influence on the selection of framing materials. Both theavailable finishes and the slenderness of the frame are factors.

� Window frame constructionWindow frame construction is governed firstly by the type of framing material and secondlyby the style of the window. The following are typical cross sections through window frames:

- TimberThere are no timber systems but there have been standardised designs. Timber ismachined to a profile from hard or softwood and joined by tenon joints and finger joints toproduce glazing frames. Today timber windows are available factory-painted and glazed.

- SteelHot rolled sections have traditionally been used to make window and door frames, Figure3.3. They are of welded construction and are robust but the range of sections available islimited.

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Cold-formed sections are available as proprietary systems Figure 3.4. The corners arenormally cleated rather than welded. Care should be taken that frames are not racked orotherwise distorted during installation as the corner joints may be damaged.

- AluminiumAluminium can be extruded in an infinite number of complex shapes and to very closetolerances Figure 3.5. Window and wall framing systems consist of a number of profiles tofacilitate extrusion and assembly.

Aluminium profiles are formed into frames by the use of mechanical joints. Window framescomprise a main framing member that provides the strength and stiffness and an extrudedglazing bead that generally clips into place to retain the glazing in a drained glazing rebate.

Aluminium frames are thermally broken to make them more energy efficient and reduce therisk of condensation forming.

- PVCuPVCu framing members are formed into window frames either by heat welding the membersat mitred joints or by mechanical joints. Heat welded joints are more common and provide aclean seal that keeps water out of the frame. Window frames comprise a main framingmember that provides the strength and stiffness and an extruded glazing bead that generallyclips into place to retain the glazing in a drained rebate, Figure 3.6.

Many window suppliers are now able to supply an additional outer frame of galvanised steel. This canbe built into a new wall allowing the window to be fitted sometime after the bricklaying has beencompleted. - CompositeFrame construction depends largely on the material of the main or central element. Forinstance a timber window is made and then clad with plastic or metal. The jointingtechnology has to take account of the materials to be joined and the presence of differentmaterials and is generally more complex than for non-composite frames.

� DoorsDoors are constructed from all of the framing materials. In general doors are made fromlarger sections. This is due to their size but also due to the robustness requirements,particularly for commercial buildings. The most commonly used framing materials arealuminium, hardwood and PVCu.

� TolerancesOverall tolerances for windows and doors are set out in the British Standards for eachframing material. Tolerances are defined in terms of height, width and difference betweendiagonals (or squareness). They are (in mm):

Material Width Height Diagonal

TimberSteelAluminiumPVCu

� 2�

1.5�

1.5� 3

� 2� 1.5� 1.5� 3

3,5 or 10+444

+depending on size of window

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Particular manufacturers will be able to make windows to greater accuracy. However theymay not be able to do so for very large windows. The tolerance achieved with a compositeframe should be the greater of the above when considering both materials.

The squareness of a fixed frame may change if it is fixed incorrectly to the wall. That of anopening frame may change as it is glazed. Squareness should be checked before and afterinstallation.

The rigidity of a window frame depends on the presence of the glazing and the positioning of thesetting blocks. The use of factory glazed windows can overcome this problem. However care is stillrequired with the frames of doors and opening lights.

� Curtain wall sectionsStick system curtain walls comprise mullion (vertical) and transom (horizontal) framingmembers. Curtain wall frames act structurally to resist wind loading and to carry the weightof the wall. A typical profile is shown in Figure 3.7.

The profile comprises an outer section that serves to hold the infill material in place, preventwater penetration and form an air seal. The inner section comprises a hollow structural boxthe depth of which determines the strength and stiffness of the section.

Most curtain walls are constructed from aluminium profiles. Some walls are constructed asan assembly of windows with PVCu frames. These are supported in a structural frame themullions and transoms of which are aluminium sections sheathed with PVCu. Stick curtain wall members are delivered to site machined and cut to length. A high degreeof accuracy is required in cutting to length. Slight variations in the length of members willresult in the erected frames being out of square or distorted, whereas if all the elements areconsistently over or under size the frame can be erected square, but the final bay may haveto be manufactured specially to fit the remaining gap. The tolerance for these elementsshould be agreed at the design stage. Framing members may be pre-assembled as ladderframes or unitised walling.

Framing members may be designed to retain the infill panels in a number of ways:

- Pressure capThe most common means of retaining glazing in a curtain wall frame is by using a pressureplate which secures the glass in the glazing rebate around the full perimeter of the glazingunit.

Pressure caps are secured in position by screws which must be either tightened to a required torqueor to a stop where the pressure cap makes contact with the frame.

- Structural silicone glazingStructural silicone provides a means of retaining glass without the need for externalcomponents. It is therefore possible to obtain a smooth façade.

It is important that the structural silicone should be applied under controlled conditions in afactory. This should ensure a clean environment and controlled curing times.

To achieve this the structural silicone is normally used to attach the glazing to a carrier framethat is then fixed to the curtain wall frame using mechanical fixings.

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- Bolted connectionsBolted connections have been developed as an alternative means of achieving a smoothfaçade. Bolted connections can be used with glazing units and single glass.

� Rainscreen frames and railsRainscreen is a layered form of construction comprising an outer cladding or rainscreen, acavity and a backing wall. Rainscreens may be constructed in various ways. Panels may besupported by a masonry or concrete backing wall via brackets or timber battens.Alternatively the rainscreen panels may be supported by rails spanning between floors or aframe consisting of vertical and horizontal members.

The frame may be of similar proportions to a curtain wall frame and span from floor to flooras a self contained, integral, rainscreen. Alternatively sections of lighter weight may be usedattached to a background wall for support.

Frame members are made from aluminium profiles or cold formed steel sections. Thetolerances on components are similar to those achievable for curtain walls.

Figure 3.1 Window types

Figure 3.2 Window types

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Figure 3.3 Hot rolled steel window frame

Figure 3.4 Cold formed steel glazing frame

Figure 3.5 Aluminium window frame

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Figure 3.6 PVCu window frame

Figure 3.7 Aluminium curtain walling frame

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4 Gaskets

� FunctionGlazing gaskets are required to:

- Limit air leakage and water penetration- Allow relative movement- Distribute and absorb loads- Accommodate tolerances

� MaterialsThere is a very wide choice of gasket materials available to the designer. Materials areselected for their ability to:

- Retain their shape- Resist weathering- Work at extremes of temperature- Resist tearing

Cost is also of course a consideration when selecting a suitable gasket material.

Materials used to make gaskets can be grouped into families but within each family a widerange of performance can be achieved. It is wrong to assume that all gasket materials arethe same because they are in the same family. Gaskets from one supplier should not bereplaced with those from another without considering the performance requirements givenabove.

The most commonly used gasket materials can be grouped into the following families:

EPDM Neoprene Shape retention Good Shape retention Average Low temperatures Good Low temperatures Average Tear resistance Good Tear resistance Very good Weathering Good Weathering AverageCost Average Cost Average Silicone Butyl Shape retention Good Shape retention Poor Low temperatures Very good Low temperatures Good Tear resistance Poor Tear resistance Average Weathering Very good Cost Expensive

Thermos-plastic rubbers HypalonShape retention Poor Shape retention: AverageLow temperatures .. Low temperatures AverageTear resistance .. Tear resistance GoodWeathering .. Weathering ..Cost Average Cost …

� Types Gaskets are made in a range of shapes and sizes as shown in Figure 4.1 and can becategorised in several ways as follows:

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Type of sealA weatherstrip is a gasket whose primary purpose is to prevent water entering a joint andwhich will normally be located on the exposed side of the joint.A draughtstrip is primarily intended to prevent the passage of air through the joint and isnormally located at the back of the joint.

Method of fixingThree methods of locating gaskets are employed:

- Push-in gaskets are designed to be fitted into a groove in the mounting surface, prior tothe formation of the joint.

- Drive-in or wedge gaskets are designed to be forced into the gap between the mountingsurface and contact surface, usually as the last stage in sealing the joint. A drive-ingasket can usually be removed by pulling it from the joint, although it may bemanufactured with a rigid strip that makes this difficult.

- Slide-in gaskets are designed to slide into a groove on the mounting surface, but mustbe installed from the end of the groove. A slide-in gasket can usually only be removedby sliding it out from the end of the groove.

- Slide in gaskets can only be installed as single lengths and corner joints have to bemade after installation. Factory made joints perform better than site made joints.

Principle of operationMost gaskets form a seal as a result of compression of the bulk material but some gasketsform a seal by deflection, either of a cantilevered arm or a hollow tube and others work bywiping contact with minimal deflection.

To seal effectively a gasket must remain in compression however the compression of thegasket will cause forces to be exerted on the contact surfaces of the joint. The joint musttherefore be designed to ensure that when the joint is at its widest there is sufficientcompression in the gasket to create an effective seal. However the gasket must also becapable of being compressed sufficiently to fit when the joint is at its narrowest in such a waythat the forces on the contact surfaces do not damage the joint components or preventmovement.

Where a single gasket cannot accommodate the full range of possible joint widths due tomanufacturing and erection tolerances, it may be necessary to have a range of gaskets available.The installer can then select the appropriate gasket by measuring the width of the joint gap.

The force exerted by a gasket in compression will gradually decrease over a period of timedue to the effects of creep and stress relaxation. There will also be a reduction in recoveryof compression when the load is removed.

Corners Gaskets are either injection moulded or extruded. Most glazing gaskets and other gaskets used in thefacade are extruded as continuous lengths. At corners the gasket has to be cut and joined. The practice of bending the gasket around the corner is generally unacceptable as the cross sectionof the gasket distorts locally to the corner. The following options are available for making corner joints:- Cut extrusions to length and glue

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- Cut extrusions to length and heat weld- Cut extrusions to length and site vulcanise- Mould corners and bond to extrusions- Mould corners onto extrusions All of these methods will produce a single gasket that forms a continuous seal around the infill panelor glazing. This is recommended for the inner (air) seal of a curtain wall, Figure 4.2. For window glazing and for the outer (water) seal of a curtain wall it may be acceptable to mitre thecorners of the gasket and make an unbonded butt joint at each corner.

� InstallationIt does not matter how much effort is expended in designing the perfect joint and the perfectgasket if it is then installed by an untrained workforce with little appreciation of theperformance requirements of a sealed joint.

Basic good practice includes:

- Careful handling of the gaskets to avoid damage- Cleaning of joint surfaces including removal of swarf. Lubricants may be used to ease

fitting of gaskets but must be compatible with the gaskets and adjacent materials.- Leaving gaskets unpacked in a warm environment to relax and recover their natural

shape prior to installation is also recommended although this may leave the gasketsprone to damage.

- The gasket should be inspected before installation and discarded if visible defects suchas cuts and abrasions are found.

Temperature may affect the flexibility of the gasket and width of the joint. Generally it is notrecommended that gaskets are installed at temperatures below 5�C and even at thistemperature the joint may have opened up due to thermal contraction of the components,leading to the risk of crushing the seal at higher summer-time temperatures. The correct gasket should be used. The size of gasket to be used depends on the framedimensions and thickness of the glazing unit or infill panel. Different sizes of gasket may beavailable to accommodate different glazing types and tolerances.

Gaskets that are undersize and easy to insert will not be compressed and form a proper sealthroughout the life of the wall. Gaskets that are too tight and are forced into position maycrush the edge of the infill.

Gaskets that are stretched as they are fitted will return to their original length after installation leavinggaps at any butt joints. Gaskets should be cut slightly over size and compressed lengthwise as theyare fitted. Fitting commences from the ends followed by the middle, Figure 4.3. Gaskets areavailable with co-extruded cords that prevent stretching of the gasket. Gaskets should not be twistedor folded during fitting.

Most glazing systems are designed to be dry glazed using only gaskets. However somesystems require the use of a sealant with the gasket. This need arises with special systemssuch as blast resistant glazing. This should be done in accordance with the systemdesigner’s recommendations. The arbitrary use of sealants in combination with gasketsshould not be allowed.

Like sealants, gaskets are a target for cost cutting. A fabricator will buy cheaper gasketsfrom another supplier just to save a few pence on the cost of each metre length, without anyform of guarantee that the new gaskets will perform satisfactorily. The cost of even a small

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amount of water leakage, in terms of problem rectification/damage repair never justifies theinitial cost saving, but the capital cost saving is made by the fabricator, who rarely sees theclients’ costs of repair.

Figure 4.1 Types of gasket

Figure 4.2 Moulded gasket corner

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Figure 4.3 Sequence of fitting a dense gasket

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5 Sealants

� FunctionSealants are used to make water seals, air seals, combined seals or to protect an internalseal. They have to adhere to the materials they connect, resist tearing and be durable. Inmovement joints they should not stress adjacent materials.

Many sealants are likely to have a shorter useful life than the design life of the buildingunless they are protected and provision should be made for replacing the sealants within thejoints, or oversealing.

� Sealant systemsSealants used in modern façade construction are wet applied materials based on syntheticpolymers which cure to form flexible solids. Oil based mastics which gradually harden withtime are not generally suitable for use in these applications.

Sealants should be used as part of a system comprising sealant, cleaner, primer and backerrod.

SealantThere is a large range of wet applied sealants. Supplied in tubes or tins, they are either oneor two part materials. One-part materials avoid the need for site mixing but generally takelonger to cure as they cure from the surface.

CleanersCleaners are used to prepare surfaces before a primer or sealant is used. They are used todegrease the surface and are normally solvent based.

Some cleaners are not suitable for use with all materials, particularly plastics. Cleanersshould be chosen to be compatible with both the sealant material and the substrate.Cleaners should be tested on a small area of substrate before being used more widely. PrimersPrimers are used to prepare the surfaces the sealant has to bond to. They may seal thesurface to prevent penetration of the sealant and improve bond or they may promote achemical bond between the sealant and substrate material.

Primers used to seal porous materials serve to reduce seepage of the sealant into thesubstrate and possible staining of surfaces adjacent to the joint.

Backer rodsBacker rods are used with wet applied sealants to control the joint shape and to limit thewaste of sealant material in joints that are too deep.

Sealing stripsAn alternative to wet applied sealants is to use sealing strips. Sealing strips are flexiblematerials which are pre-formed in a range of sizes and sections and mainly rely oncompression although some adhesion to a joint face may take place. They may beconsidered as a special type of gasket and are of two basic types:

- Mastic strips, usually manufactured from relatively soft, tacky synthetic rubber to whichan easily removed backing paper is applied; and,

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- Cellular strips, usually based on a synthetic polymer, which may also be edge-coatedwith an adhesive layer. They may be composed of closed cell material or open cellmaterial impregnated with a sealant. They are supplied pre-compressed to about 20% oftheir normal thickness and expand after placing. They can either be inserted in apreformed joint or fixed to one side of the joint before placing the component forming theother side of the joint.

� Types of jointJoints are made to join together elements of the building and may be used for two purposes:

Fixed jointsThese occur where materials are joined because maximum panel or unit size requires theuse of more than one element. Joints also occur where different materials or componentsmeet.

At a fixed joint the adjacent components are fastened together to prevent movementbetween them. The joint then has a constant size and shape and the sealant does not haveto move.

For fixed joints the materials used are selected to be durable, Figure 5.1 and to bond to thesubstrates.

Movement jointsThese joints are provided to allow the building and the cladding to move. Movement occursbecause of temperature change, wind loading and imposed loading amongst other things.Movement joints are made at the natural joints in the building where there would otherwisebe fixed joints.

The shape and size of a movement joint will change daily and over longer periods of time. Asealant that can move in the required way is chosen for a movement joint and there is a widerange of performance available Figure 5.2. Sealants also have to be durable and bond tothe substrates.

In a movement joint the stretching of the sealant will make greater demand on the bond tothe substrate.

Joint size The exact size of a movement joint gap, Figure 5.3 is important to its short and long termperformance. If the width of a movement joint is made only half of the intended size then the forceswithin it will be double those intended and failure is almost inevitable. All joint designs should specifya minimum joint gap size to be achieved on site.

� Joint shape There are three basic shapes of sealant joint: Butt jointThis shape of joint occurs when two thick panels are joined edge to edge or where thinnerpanels are required to have a flush face Figure 5.4.

Thin panels should be formed with a return that gives an adequate bond area for the sealant.

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It is important, particularly for a movement joint, that the sealant material can stretch across its fullwidth. A backer rod or release tape is used at the back of the joint to prevent adhesion of the sealantat the back of the joint. If the sealant is not free to move then it will tear early in its life Figure 5.5.

It is important to control the depth of sealant within the joint. Too deep a sealant will causehigh stresses and tear the sealant or adjacent material. It will also be wasteful of material.Too little sealant will not create a robust joint. Lap jointThis shape of joint is most commonly used for fixed joints although it can be designed tomove.

It is important that the gap achieved on site is not less than that intended, particularly for amovement joint. Otherwise the sealant will be overstressed leading to tearing or debonding.

Backer rods should be used to control the size of the sealant bead within the joint to avoidthe wasteful use of material and to provide a robust joint Figure 5.6.

Fillet jointThis shape of joint is frequently used when components are neither lapped nor positioned togive a flush face. This is the joint commonly used to seal windows recessed in openings.

The joint should be constructed to give an adequate contact area between the sealant and thesubstrates. This should be not less than 6mm onto a non-porous surface and 10mm onto a poroussurface. A fillet joint made in front of two components that are very close together will tear Figure 5.7and a minimum gap of 5mm should be allowed.

Joints between windows and walls are not designed as movement joints but are not perfectlyfixed and so some movement will occur.

Backer rods should be used to prevent the wasteful use of material and so that the joint canbe properly tooled to form a good bond.

� Materials Sealants are commonly classified by their base materials: - Silicones- Polysulfides- Polyurethanes- Acrylics

However the performance of a sealant is not only governed by the base material but also byadditives such as plasticisers, retarders and fillers. The preferred practice adopted by recentBritish Standards is to specify sealants by performance.

The following classification system is given in BS ISO 11600:

Sealant typeSealants may be classed as type G which are suitable for use in glazing and type F whichare suitable for use in building joints other than glazing.

Sealant classFour classes are given relating to the amount of movement the sealant can accommodate.The classes are 7.5, 12.5, 20 and 25 which give the allowable movement as a percentage ofthe unstressed width. Sealants can accept this movement in both compression and tension.

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Although a class 25 sealant can accommodate more movement than a lower class sealant itwould only be suitable for use in place of a lower class sealant if all the other properties ofthe sealant are also acceptable.

Some sealant specifications give movement accommodation as the total movementexpressed as a percentage of the minimum joint width (the joint width when the sealant isfully compressed). This will give values about twice those given using the BS ISO 11600definition. When selecting sealants for movement joints it is important to check the basis onwhich the movement characteristics sealant are given.

Sub-classesSub-classes relate to the elastic properties of the sealant.Class 20 and 25 sealants are elastic and may be designated LM for low modulus or HM forhigh modulus.Class 7.5 sealants are plasticClass 12.5 sealants may be designated P for plastic or E for elastic

Test criteria are given in British Standards to establish compliance of the sealants with thisclassification system. This classification system gives a starting point for the specificationand selection of sealants however other properties which must be considered include:- Life expectancy- Colour- Compatibility with substrate- Adhesion to substrate- Stress relaxation

It follows that sealants should not be casually chosen or substituted at site. It will always bepossible to buy a cheaper sealant but it will probably not be suitable.

� Storage and useA successful sealant joint requires correct installation procedures.

All materials making up the sealant system must be compatible and should preferably comefrom the same supplier. The materials making up the sealant system must also becompatible with the substrate.

Materials must be used in accordance with the manufacturer’s instructions. The provision ofdetailed site-specific method statements ensures that the applicator is aware of the correctprocedures and allows co-ordination of sealant application with other work on site. Aspectsto be included in the method statement are described below.

StorageSealants and associated materials including primers and cleaners may contain hazardousmaterials and require appropriate storage conditions. Materials may also require protectionagainst frost and excessive heat or humidity during storage. Storage procedures shouldalso ensure that materials are used before their expiry dates.

InspectionBefore sealant application commences joints should be inspected to ensure that theirdimensions are within permitted limits and that the adjacent materials are of suitable quality.The inspections should be carried out in sufficient time to allow remedial work to be carriedout where necessary.

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WeatherTemperature will affect the properties of the sealant and the opening of joints. In coldconditions the sealant will be more viscous and take longer to cure whereas in hot conditionsit will be less viscous and have a shorter working life.

Sealant application is normally limited to temperatures between 5�C and 40�C. Thesetemperatures apply to the surfaces to be sealed not the ambient air temperature.

Frost may persist on shaded surfaces after the air temperature has risen to 5�C andsurfaces subject to direct sunlight may reach temperatures as high as 80�C.

It is also necessary to consider likely temperature changes during the curing period of thesealant as excessive movement during this period may cause the joint to move while it iscuring and pucker the cured surface of the joint.

Sealants should be applied in dry conditions although some primers are tolerant of dampsurfaces. Wet surfaces can arise due to condensation in cold weather as well as rain. Forthis reason sealants should only be applied when surface temperatures are at least 5�C andrising.

CleaningCleaning of the joint surfaces is always necessary. The cleaning methods to be used varyaccording to the type and condition of the surfaces.

Physical removal of dirt may require use of a dry brush, compressed air, wire brush orabrasive pads. The method chosen must ensure that the surface is not damaged.

Removal of grease may require use of a solvent. The solvent must be compatible with thesubstrate, primer and sealant and must be clean. Cloths used for application should also beclean and lint free: use of white or light coloured cloths is preferable so that soiling is evident.One cloth should be used to apply the cleaner and a second to wipe off.

MaskingMasking tape is useful on substrate surfaces where removal of excess sealant is difficult andmay also be used to improve the appearance of the finished joint by giving a clean edge.Tape should be applied prior to application of primer and the tape should not touch surfacescleaned for sealant application. Tape must be removed immediately after sealant applicationand tooling.

PrimingThe need for a primer will depend on the substrate and sealant to be used. Non-poroussurfaces usually use a silane type primer which must be applied sparingly using a cloth.Resin type primers are normally used for porous surfaces and may be applied by brush orcloth.

Primer should only be applied to the sides of the joint to which the sealant is required toadhere.

Care should be taken to avoid contamination of the primer both before application andbetween application of primer and sealant.

The primer must normally dry or cure before application of the sealant but if left too long maycease to be effective.

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Back up materialClosed cell polyethylene or polyurethane foams supplied in rods, hoses or flat sheet, whichmay be cut to form rectangular sections, are commonly used as back up materials to controlthe depth of the joint. The back up material may have a surface skin which preventsadhesion of the sealant. If this is only present on one surface care is required to ensure thatthe material is inserted the right way round. If the back up material does not have a surfaceskin a bond breaker tape is required. Polyethylene and PTFE are commonly used for bondbreaker tape.

The back up material may be applied before or after priming. In the former case care isrequired to ensure the primer is not removed or damaged during installation of the back upmaterial and in the latter care is required to avoid application of the primer to the back upmaterial.

Foam back up material should be compressed by 25 to 50 % when installed to ensure that itis held securely in place during sealant application. The backer rod must be placed carefullyto avoid distortion or twisting and it must be at the correct depth as it controls the depth ofthe sealant.

If the backer rod is damaged during installation gases can be released and as a precaution aperiod of 30 minutes should be left between installation of the backer rod and application ofsealant to allow gasses to disperse. If severe damage to the backer rod occurs replacementis necessary.

MixingTwo part sealants require mixing. Mixing is normally carried out using a paddle in a lowspeed drill.

Mixing needs to be thorough, indicated by a uniform colour, but if too vigorous air can betrapped in the sealant.

Curing of the sealant will commence as soon as it is mixed hence it should be mixed inquantities which can be used within the pot life.

Sealant applicationSealant is normally applied from a hand-operated gun.

The nozzle should suit the width of joint and the rate of extrusion and movement of the gunshould be such that the joint is filled with sealant in a single pass Figure 5.8.

For very wide joints it may be necessary to use several passes of the sealant gun buildingup from the back corners of the joint.

ToolingTooling removes voids, improves adhesion by compacting the sealant against the sides ofthe joint and gives a neat finish. A slightly concave surface reduces movement stresses butovertooling can leave the sealant too thin at the centre.

Tooling must be carried out before the surface forms a skin which may be damaged. Theavailable time for tooling varies from a few minutes to several hours depending on the typeof sealant and ambient conditions.

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Tooling is usually carried out using a wooden or metal spatula which may be wetted withwater or a dilute detergent solution. Water should be used sparingly and applied to the toolrather than the joint. Excess water should be shaken from the tool.

ProtectionDuring the curing cycle dirt may adhere to the tacky surface of the sealant and becomeembedded. The sealant should therefore be protected from dirt and debris. The sealantmay also require protection against inclement weather. However sealants may require thepresence of air, moisture or UV to aid curing and protection should not interfere with thecuring process.

Figure 5.1 Durability of sealants

Figure 5.2 Movement accommodation of sealants

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Figure 5.3 Joint width

Figure 5.4 Joints between thin panels

Figure 5.5 Use of release tape

Figure 5.6 Use of backer rod

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Figure 5.7 Fillet joint minimum dimensions

Figure 5.8 Use of sealant gun

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6 Finishes

� FunctionMany facade materials have to be finished or coated to protect them from the environmentand give the required appearance. The quality of the finish may be the greatest factoraffecting the useful life of the wall and is likely to be a contentious issue if the appearance isnot acceptable to the client.

Materials may rot, corrode and suffer other forms of degradation in the presence of moisture,UV, salt laden air and air borne pollutants. The materials most in need of protection fromthese atmospheric conditions are metals and timber. Plastics and other materials may bepainted for reasons of appearance.

� Aluminium Mill finishAluminium may be left uncoated as ‘mill finish’ aluminium. In this form the surface oxidisesto form a stable coat. However the oxide coating appears as a slightly white bloom that maynot be visually acceptable.

Although the oxide coating is stable it will penetrate under adjacent paint and powderfinishes allowing them to blister and separate.

CoatingsCoated aluminium is a durable material and a useful life of 25 years or more can beachieved. The quality of the paint or powder finish depends on the materials used and thecleaning and pre-treatment of the aluminium prior to painting.

Paints and powder coatings are applied to closely controlled thicknesses in the range 40-100microns. The coating is then oven baked to achieve a uniform and durable surface.

Finished aluminium is a quality product that cannot easily be repaired on site. It should betreated with care and protected as necessary during construction.

The commonly used coatings are:

- Polyester powder coating- PVDF (Polyvinylidene Fluoride)- Wet applied paints

AnodisingAluminium may be anodised to form a hard resistant oxide coating. This coating is integralwith the aluminium but should be treated with the same care as painted and powder coatedsurfaces.

Anodised aluminium may be coloured or clear. Clear finishes are used to give corrosionprotection and should be treated with the same care as coloured surfaces.

Cut edgesAluminium is often finished in lengths prior to cutting and fabrication. Cut edges can be thestarting point for corrosion and some contracts do not allow the use of pre-finished (post-cut)aluminium.

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The risk of corrosion occurring at cut edges depends on the quality of cutting, standard ofpre-treatment and coating. The use of hand held saws and drills is unlikely to give asatisfactory edge quality. Factory machining uses separate drills and blades for workingaluminium and steel.

ProtectionAll significant surfaces should be protected from abrasion, scuffing and other damage duringtransportation and installation.

Protective tapes are used on coated aluminium surfaces but they are no substitute forcareful handling. Additional methods such as protective boards may be used to protectagainst damage by following trades.

Only low tack tapes should be used as agreed by the manufacturer. Tapes should not beleft in place for more than six months or difficulty may be experienced in removing them.

Tapes should be removed by peeling. If this is difficult a soft tool should be used. Sharpblades and solvents should not be used.

Products such as windows may be protected during transport to site by using bubble wrap,shrink-wrap or card. Tape should still be applied to protect significant surfaces during andafter installation.

Mortar drops and similar alkaline materials are particularly damaging to coatings and paintfinishes which should be appropriately protected.

Remedial workSite repairs to finishes should be agreed with the Client’s agent. It is seldom possible toachieve a repair that looks good and the Architect may ask that the component be replaced.This decision must depend on the extent of damage and any disruption that may arise.

Repairs to coated surfaces should be carried out in accordance with the suppliersinstructions. This often requires the use of specialist paint contractors.

� SteelAll steel has to be finished to protect it from corrosion. Steel may be coated in the same wayas aluminium but these finish coats are applied over a protective treatment. For use infacades steel sections are hot dip galvanised, or equivalent. This is done after machining toavoid edge corrosion. Paint or powder coat is then applied to give the required appearance.

Galvanising deposits a zinc layer on the steel, which protects the steel by forming a barrierbetween the environment and the steel surface. The zinc layer will corrode unless protectedby a coating but corrodes more slowly than steel.

Zinc also provides protection to the steel by corroding preferentially to the steel at breaks inthe zinc layer. This process is a form of cathodic protection but is only effective when asufficient area of the zinc is exposed.

Where there is a paint finish on the zinc surface, protection only occurs at small scratches.Larger areas of damage to the galvanising should be made good with zinc paint.

Protection of finishes and repair of any damage should be dealt with in the same way asdamage to finishes on aluminium.

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Steel may also be plastic coated. This finish is used for metal coil that is subsequentlyformed into profiled metal sheets or flat cladding panels. The plastic coating is applied at thesteel mill before the metal is shaped and cut and no attention is given to machine cut edges.

Holes and cut edges made with hand held tools will not have such clean-cut edges and maybe sites for early corrosion.

� TimberTimber is treated and then finished to prevent the onset of rot and provide a goodappearance. The finishes most commonly used are paints and micro-porous stains. Timberwindows are often pre-finished at the factory but may be supplied primed for painting on site.

Exposure of untreated timber to sunlight will adversely affect the durability of paint finisheshence untreated timber should be painted, or at least primed, as soon as possible.

Most timber finishes can be repaired by site painting but it is difficult to conceal heavydamage to stained timber. Timber frames should be treated with the same care as otherfinished materials.

� PlasticsMany plastic components are made from self-coloured plastic, predominantly white althoughbrown and other colours are available. These plastics cannot be refinished. They should beprotected and treated with care. Components may be coloured by co-extruding a colouredouter layer of the required colour. Damage to this layer may allow the base material colourto show.

Low tack protective tapes should be used on all significant surfaces even if they are onlyself-coloured plastic surfaces. Tapes should be peelable and used in accordance with themanufacturers' instructions. Plastics are easily damaged by solvents and some adhesives.

Plastics can be finished by painting or applying foil to the surface. Adhesive foil is commonlyused to achieve a wood grain effect on domestic windows. Repairs to painted and foiledsurfaces are difficult to achieve with any degree of success and the manufacturer should beconsulted before any remedial work is started.

� AppearanceFinishes determine the appearance of the completed building and this is a subjective issue.It is little wonder that the appearance of finishes is so often questioned. Appearancedepends on:

- Colour match- Level of gloss- Texture

On larger contracts it is common practice to make samples showing the acceptable colourrange and level of gloss. All oven-baked finishes will suffer some orange peeling and thistexture is to be expected. Samples will show the acceptable limits of this texturing. In somecases an independent inspector will be employed to acceptance test the finishes.

In either case it is advisable to gain acceptance for the finishes before they are delivered tosite or at least as the components are installed.

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When inspecting finishes for appearance they should be viewed from a distance of onemetre using normal, corrected vision in diffuse daylight.

� Cleaning down Protective tape and other protective measures should be left in place as long as possible. If tape isremoved for inspection it should be replaced, if necessary with new tape of the same type.

On completion surfaces should be cleaned down using water containing a mild detergent.Neither abrasives nor solvents should be used on any finished surfaces.

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7 Glass

� TypesGlass is available in many types, thicknesses, patterns and finishes. The glass is selectedfor reasons of safety, appearance and the way it controls the internal environment of thebuilding.

Glass may be grouped into categories by considering:

- Strength and safety- Appearance- Environmental control

Strength and safetyGlass in a building will be subject to mechanical loads in the form of wind load and impact. Itmay also be subject to stresses due to environmental conditions such as temperaturechanges. The strength properties of glass can be varied by increasing the thickness, heattreatment and combining the glass with other materials to form composites. The strength ofglass must be sufficient to resist the loads it is likely to be exposed to. Safety of glass isrelated to its strength but also takes into account the risk of injury from the failed glass.

- Annealed glassAnnealed glass is the basic form of glass produced in float glass plants. It has no specialproperties of strength or safety and on breaking it forms large shards with sharp edges.

- Heat strengthened glassAnnealed glass may be strengthened by controlled heating and cooling. Heat strengthenedglass is not a safety glass but is roughly twice as strong as annealed glass. When brokenit behaves like annealed glass and breaks into large shards with sharp edges.

- Wired glassWired glass fractures in the same way as annealed glass but remains in place with theshards held together by the wire mesh. Wired glass is not stronger than annealed glassbefore failure. After failure the strength of the pane will depend on the thickness of thewires. Wired glass is available as ordinary wired glass and safety wired glass whichcontains stronger wires.

- Toughened glassAnnealed glass is toughened by heating it to 650oC and rapidly cooling the surfaces. Thiscompresses the surfaces and increases the strength of the glass. Toughened glass isroughly five times as strong as annealed glass.

An important property of toughened glass is the way in which it breaks. Any cracking of theglass leads to a rapid release of the surface compression and toughened glass alwaysbreaks into small pieces of glass Figure 7.1. Toughened glass complying with BS6206 is asafety glass.

Toughened safety glass should be kitemarked and installed with the kitemark visible.

Toughened glass cannot be cut or drilled after toughening and must therefore be cut to sizebefore toughening.

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- Heat soaked toughened glassToughened glass may fail due to the presence of nickel sulfide crystals in the glass. Toreduce the risk of nickel sulfide failure, the glass may be subjected to a process known asheat soaking. To be effective the heat soaking process must be strictly controlled.

- Laminated glassAnnealed, heat strengthened or toughened glass can be laminated in any combination tomake a safety or security glass. Two or more pieces of glass are laminated together to givethe required properties. The glass may be laminated as a sandwich with a layer(s) ofpolyvinyl butyral (PVB) between the sheets of glass. Glass can also be laminated bypouring a resin between two sheets of glass. PVB laminates are best suited to flat glasswhile poured resins are best suited to curved glass.

Laminated glass is not as strong as a single pane of glass of the same type and thicknessbut after failure the broken pieces of glass will be held together by the interlayer.

The performance of a laminated glass depends on the type of interlayer used. Some aredesigned to resist penetration and others solely as safety glasses.

- Tempered glassTempering is the American term for strengthening and toughening. Tempered glass isroughly equivalent to heat strengthened glass and is not a safety glass. Only fully temperedglass has similar properties to toughened glass. Fully tempered glass used as a safetyglass should conform to BS6206.

- PlasticsPolycarbonates are sometimes used as glazing materials. They are used for safety glazingas they are less prone to breakage. Plastics are more flexible than glass of the samethickness. They may be sprung out of a glazing frame and are not always suitable assecurity glazing. Plastics are less scratch resistant than glass.

Appearance - PatternedGlass may be patterned by rolling a relief onto one surface while it is still hot and soft. Thisis done to obscure vision or to change the appearance of the facade. Patterned glass hasthe same strength and safety characteristics as annealed glass and is not normally a safetyglass however it is possible to toughen patterned glass. Some patterned glasses - thosethat do not have deeply embossed surfaces - may also be laminated.

- PrintedIt is possible to print patterns on to glass. This may be done to make people aware of theglass for safety reasons. In this case the patterning has to be in the correct position. Notethat company logos and other signage may be used for this safety purpose.

- Fritted and etchedThe surface of glass may be etched or otherwise altered to achieve the same effect asprinting. Again this may be done for safety reasons.

Environmental controlEnvironmental control glasses are used to limit the heat and light passing through a window.

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- TintedGlass may be tinted to reduce light transmission and prevent glare within a building.

- CoatedGlasses are coated to change the properties of the glazing. Coatings are used to reflectlight and/or heat. Increasing the amount of reflected light may be required for aestheticreasons (giving a mirror effect) or to restrict the view into the building. Reflection of heatmay be required to reduce solar gain or to retain heat within the building. The type ofcoating will differ depending on its purpose.

Low emissivity (low-E) coatings are among the most widely used and are provided to reflectheat from inside the building back to the inside and therefore reduce heat loss. They do notreflect solar radiation in the same way due to the different wavelength. They are not visibleto the eye.

- PrintedPatterns may be printed or etched onto the surface of the glass to obscure vision or preventglare.

- Double and multiple glazingGlass is frequently used as insulated glazing units (double glazing). This is normally done toreduce heat loss from the building but it can also help to reduce noise levels inside abuilding. In some cases triple glazing is used to reduce noise levels or further reduce heatloss.

Insulated glazing units may be made using any of the glasses described above and differentglasses may be used for the inner and outer panes. The panes are separated by a spacerbar. The units may be constructed with a primary airtight seal between the spacer bar andthe glass and a secondary structural seal outside the spacer bar holding the glass panestogether Figure 7.2. Alternatively a single structural and air tight seal may be used Figure7.3.

- Gas filledInsulated glazing units may be gas filled to reduce energy loss through the window. Anyunits that are broken or damaged should be replaced with equivalent units.

� Safety glazing and fire rated glazing The Building Regulations make specific requirements for the use of safety glazing and firerated glazing under certain circumstances. The design of the facade will have taken accountof these requirements. It is essential that safety glazing and fire rated glazing are installedas specified.

Safety glazingGlass in critical locations (adjacent to doorways and pedestrian areas and in windows withlow sills) has to comply with part N of the Building Regulations Figures 7.4 and 7.5.

The glass has either to break in a safe manner or resist impact. It is normal to usetoughened, laminated or wired glass in these locations. Plain annealed glass may be usedprovided no single pane exceeds 0.5 m2 in area, the smaller dimension is no more than250mm and the thickness is not less than 6 mm.

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Substitution with glass of different performance in a critical zone may be unsafe and shouldonly be approved by the specifier.

Fire rated glazing systemsFire rated glazing systems will have been tested to show that they can resist fire for therequired period of time.

The performance of a fire rated screen depends on the exact replication of the test sampleon every contract. No substitution of any framing, glazing or other components is permitted.

In the UK FIRAS maintain a register of trained installers and approved specialist contractors.

CDM RegulationsThe Construction, Design and Management Regulations require all people involved in theconstruction of a building to ensure that it is safe during construction and use. Glass is apotentially hazardous material and care will be required to ensure the safety of theworkforce, occupants of the completed building and any future maintenance workers.

� Terminology The following terms relating to glazing are illustrated in Figure 7.6.

� Sight size� Pane size� Tight size� Edge clearance� Rebate depth� Edge cover� Back clearance

� Condition The performance of glass is highly dependent on its condition. The use of damaged glass orinsulated glazing units will impair the performance of the facade.

Glass should be inspected for:

SizeGlass that is undersize will not have sufficient cover in the glazing rebate. This can lead toan inadequate seal at the gasket and in the extreme loss of glass retention.

Glass that is oversize will reduce the clearance between the glass and frame which will limitthe accommodation of relative movement of the glass and frame.

If thinner setting blocks are used to accommodate oversize glass this will reduce the cavityin the glazing rebate so that the lower edge of the glass is wetted. This may lead to thebreakdown of seals of glazing units.

Ultimately the glass may not fit into the frame if it is too large.

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Surface defectsSurface defects are uncommon with float glass. However when they do occur they areclearly visible. Surface defects are an obvious source of irritation to the client.

It is good practice to check all glass for surface defects at the time of installation. It is fareasier to replace glass at this stage while the access scaffold is still in place.

Toughened glass may have a slightly rippled surface as a result of the toughening process.This is generally accepted but if particularly bad it may be unacceptable and the glass mayhave to be replaced.

If the cavity of an insulated glazing unit is at a different pressure to the surrounding air, theglass will dish and give distorted reflections.

Pressure differences can be caused by sealing the units at too high a temperature or at adifferent altitude from the site. This results in dishing of the glass as the cavity volumechanges. Visual effects can be quite pronounced and unacceptable.

Changes in weather conditions will have a smaller effect that is normally acceptable.

Edge defectsEdge defects include:

- Feathering where the edge of the glass is not exactly square to the face and may not beplane

- Venting where the edge of the glass is clearly chipped to leave sharp edges around adepression

Feathering of the edge is acceptable up to a point. Venting is never acceptable Figure 7.7.

Edge defects cause stress concentrations which weaken the glass if it is subject to load.Thermal fracture of glass takes place if there is a large temperature difference betweendifferent parts of the glass. This can occur when most of the glass is heated by solarradiation but the edge is kept cool by shadows or the insulation of the frame. Stressconcentrations at edge defects increase the risk of thermal cracking.

An edge tape may be used but this is not recommended as it provides little protection, hidesedge damage, prevents inspection of the seal(s) and can even trap moisture causingbreakdown of the seal.

Laminated glassLaminated glass should be visually and optically acceptable. There should be no damage tothe edge of any sheet of glass in the laminate.

Edge sealsSealed units are made with either single or double edge seals to comply with BS5713.

Double edge seals are used to give a longer life to the unit. Any units replaced on site dueto breakage or the presence of defects should be replaced with units of the sameconstruction.

Edge seals should be free of any visible air bubbles.

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� Identification Identification of glass on site can present difficulty if it is part of a glazing unit, has invisiblecoatings or particular strength properties. The main methods of identifying glass are:

- Visual inspection with a gauge card held against the surface will identify the glassthickness using the reflection from the back face of the glass. A reflected flame will appeardifferently on coated surfaces.

- Marking of glazing units at the time of manufacture assists identification. Labels shouldshow: type of glass, size, manufacturer, glazing position and orientation. Toughened safetyglass should be kitemarked to BS6206. Glazing units may be kitemarked to BS5713.

- Gauges or meters may be used to determine glass thickness. Several commercialsystems are available.

- DSR (differential surface refractometer) equipment can be used to determine thesurface stresses in glass and the degree of toughening. This equipment is expensive and isunlikely to be available on site.

- Ultrasonic test equipment can be used to identify laminated glasses. These also sounddifferently when tapped.

Suitable methods of identification:

Glass types MethodsClear float Visual or meterPatterned VisualWired VisualTinted VisualCoated Visual or meterHeat-strengthened DSRToughened Mark, DSR or polarised lightBent VisualLaminated Mark, ultrasonicGlazing unit Mark on spacerPrinted VisualOff-line coated Visual, meter, reflections

� Glass installation The following standards apply to glass installation: BS6262 Code of practice for glazing of buildings BS8000 Pt7

Code of practice for glazing on building sites

BS5516 Code of practice for design and installation of sloped and vertical patentglazing

BS8213 Pt4

Code of practice for the installation of replacement windows and doorsets inbuildings

Glazing materials should be installed in accordance with the manufacturer’s instructions andBS6262. BS6262 gives general guidance applicable to most windows.

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Where manufacturer’s instructions differ from BS6262 the manufacturer’s instructions shouldbe followed.

PositioningIt is important that glazing units are correctly positioned. Units that include safety glassshould be used in the correct openings and not swapped with non safe units.

Units that have different glasses for the inner and outer panes should be positioned with thecorrect face outermost. This may be required for reasons of safety, appearance or theeffectiveness of energy efficient glazing.

Each glazing unit will contain two or more pieces of glass that will be of slightly different sizedue to manufacturing tolerances. Good quality glazing units are constructed with all glassaligned on two edges of the unit that are labelled ‘bottom’.

Glass should be installed with the correct edge resting on the setting blocks so that allsheets of glass are equally supported.

Setting blocks and spacersSetting blocks are used to support the glass and must support both panes of a glazing unit.They prevent glass to frame contact and centralise the glazing in the frame, Figure 7.8.

Setting blocks should support the glazing clear of any water that enters the glazing rebate.

Setting blocks should not block any drainage paths. Some systems require setting blocksthat bridge the drainage channel. Use of sealant to locate setting and location blocks mayalso block drainage paths.

Setting blocks may be made from the following materials:

- Neoprene with Shore Hardness 80 to 90- Plasticised PVC with softness of 35 to 45- Extruded unplasticised PVC

Hammered lead is sometimes used in undrained systems and sealed hardwood may beencountered in some windows but should not be used in curtain walls.

Location blocks are used to prevent lateral movement of the glazing and give rigidity toopening lights and factory glazed products. They are made from the same materials assetting blocks.

Distance pieces are used to maintain the distance between the glass and the frame whenusing wet applied sealants Figure 7.9. They are made from the same materials as settingblocks.

Glass and frame supportThe glazing material stiffens the frame of opening lights and doors and prevents themdistorting or sagging in use. The setting block positions are selected to correctly stiffen theframes as well as support the glass. For windows that pivot on a horizontal axis the settingblocks at the top of the frame also support the glass.

The recommended positions for setting blocks for windows are shown in Figure 7.10 but themanufacturer’s instructions should also be read.

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Setting blocks should be at least 30mm and no more than 100mm from the corner of theglazing frame.

Curtain walling and glazing screens have to move to accommodate movement of the primarystructure. Location blocks in curtain walling should be placed near the bottom of the glass toprevent lateral movement of the glass but allow racking of the frame Figure 7.11.

Edge clearanceGlass should be fitted into the frame with adequate edge clearance. This is necessary sothat:

- The glass and frame can move without stressing the glass- Water entering the frame can drain freely

Minimum edge clearances for glass are:

- 3mm for glass sizes up to 2m- 5mm for glass sizes over 2m- 6mm for all drained systems

Minimum edge clearances for plastic glazing materials are:

- 3mm for plastic sizes up to 1m- 5mm for plastic sizes between 1 and 2m- 7mm for plastic sizes between 2 and 3m

DrainageDrain holes in the bottom or face of the frame must not be blocked by setting blocks, swarfor sealants.

� Storage and Handling Glass weightTypical glazing units are heavy and larger units require special consideration. It is alwayspreferable to glaze windows at the factory. However for larger windows the completedweight is too great to be lifted manually and these windows have to be site glazed. Somewindows have to be deglazed for fixing into the opening.

Glass weighs 2.5kg/m2/mm. Weights of typical glass products are shown below;

6mm glass 15 kg/m2

6 - 12 - 6 glazing unit 30 kg/m2

7.3 - 12 - 6 laminated glazingunit

32.5 kg/m2

15mm glass 37.5 kg/m2

Consideration should be given to mechanical handling and lifting of larger glazing units andcomplete glazed windows.

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Glass storageGlass should be stored:

- In the dry- Out of direct sunlight- Stood on edge- Protected from impact- Protected from dirt

Glass should be stored on site in a protected location where it will not be damaged and doesnot become marked or unduly dirty.

If glazing seals become wet, particularly if water becomes trapped behind edge tapes, theseals will start to break down. If water is trapped between two pieces of glass for too longthen the glass surfaces may be permanently marked.

If glass is stored in direct sunlight then heat passes into the stack and cannot escape. Theglass within the stack can become very hot causing fracture.

Glass should be stored stood on edge and inclined against a rest to prevent it from falling.With glazing units both edges should be supported to reduce the risk of edge damage. Asuitable arrangement is shown in Figure 7.12.

Figure 7.1 Fractured toughened glass

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Figure 7.2 Dual seal edge detail

Figure 7.3 Single seal edge detail

Figure 7.4 Critical areas in a glazed screen

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Figure 7.5 Critical areas in windows

Figure 7.6 Glazing terms

Figure 7.7 Edge damage to glass

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Figure 7.8 Setting block

Figure 7.9 Distance pieces

Figure 7.10 Setting and location block (windows)

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Figure 7.11 Setting and location blocks (curtain wall)

Figure 7.12 Storage of glass

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8 Brackets and fixings

� FunctionBrackets and fixings are used to attach curtain walling and windows to the supportingstructure.

Windows are normally located within the supporting wall so that vertical loads are transferredto the structure by bearing at the window cill. Fixings are required to hold the frame securelyin position and resist horizontal loads. Fixings may pass directly through the frame into thesupporting structure. Alternatively a strap or lug may be attached to the frame and fixingspass through the strap into the supporting structure, Figure 8.1. The use of lugs or straps isessential for factory glazed windows. When through fixing care is needed to avoid crushingor distortion of the frame.

Curtain walling is normally positioned in front of the supporting structure and brackets arerequired to connect the curtain wall to the structure. Fixings are then required to attach thebrackets to the structure.

� Performance of BracketsBrackets are required to perform a number of functions as described below.

LoadsVertical forces due to dead loads and horizontal forces due to live loads are transferred tothe structure by the brackets. To transfer these loads two types of connection are required:

a) Support brackets are required to carry dead loads and these will prevent vertical movement ofthe mullion relative to the supporting structure. Only one support bracket is necessary foreach length of mullion and provision of additional supports is undesirable as movement will berestricted (see below).

b) Restraint connections are required at both top and bottom of mullions to resist windloads.

Two possible bracket arrangements for a single storey height mullion are shown inFigure 8.2. The top hung arrangement is more common but the bottom-supportedarrangement may be used, particularly for low-rise construction. Where mullions span morethan one storey restraint connections are usually provided at the intermediate floors.

AdjustmentAll brackets should provide adjustment in three directions to overcome dimensionalvariations.

Means of adjustment include:

- Slotted holes for fixings These may need to be combined with serrated surfaces to prevent further movement afteradjustment or low friction surfaces to permit movement by sliding after installation;

- Site-drilling or welding after positioning of componentsThis may be used for final fixing after initial fixing with slotted holes.

It is likely to be less successful for fixings into concrete as the required hole positionsmay coincide with reinforcement.

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- Shims, packing pieces or washers If excessive thicknesses are used nuts may not engage fully with bolt threads andbending stresses may be induced in bolts.

Packing pieces may also reduce the contact area between components increasingstresses and inducing additional bending.

- Sliding connections;

- Channel fixings - Comments for slotted holes apply:

MovementsDesign of brackets needs to take account of movements of the curtain wall and structure toavoid:

- Imposing loads on the curtain wall for which it has not been designed

- Breakdown of seals due to large movements being transferred from the frame to thecurtain wall

For stick curtain walls, vertical movements are usually accommodated in the splicesbetween mullions which allow the sections of mullion to slide vertically but transfer horizontalload.

Movements which cause shear of the curtain wall can usually be accommodated by rotationof the mullion/transom joints provided there is sufficient clearance between the frame andthe infill.

Although vertical movements will normally be greater than horizontal movements, horizontalmovements must also be considered.

All brackets should allow the required amount of movement after fixing. Movementaccommodation should not be sacrificed to achieve fit of incorrectly sized elements andcomponents.

Resistance to corrosionTwo forms of corrosion warrant consideration:

- General corrosion of individual components including brackets, fixing bolts and curtainwalling

- Bi-metallic corrosion resulting from contact between components made from differentmetals.

Requirements for corrosion resistance also apply to fixings and are described in thematerials section below.

BuildabilityCladding is often erected at height in inclement conditions. Connection details shouldtherefore be simple to construct, to improve safety and reduce the risk of poor workmanship.

Brackets which are capable of being lined and levelled in advance of cladding erection canproduce overall cost benefits.

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� Fixings

Fixings are required to attach curtain wall brackets and windows to the structure. A widevariety of proprietary fixings is available.

The selection of suitable fixings for a particular application depends on a number ofrequirements including the magnitude of the loads to be carried, the nature of the loads(shear, tension or compression), thickness of the fixture (including provision for packing orshims), the substrate and the required life of the fixing. Substitution of a specified fixing byan alternative type requires a reappraisal of all these factors.

The load which fixings are required to carry varies greatly. Window frames will normally besecured with a number of fixings at intervals around the perimeter, Figure 8.3, giving fairlysmall loads at each fixing. A fixing for a curtain wall bracket will carry the total wind load fora larger area of cladding, Figure 8.4. The curtain wall fixing will also have to carry the deadload.

The load from the curtain wall bracket may be carried by one or two fixings giving little scopefor load redistribution in the event of failure whereas failure of a single window fixing may beaccommodated with little difficulty.

The performance of curtain wall fixings is therefore more critical to the safety of theinstallation.

Fixings may be required to connect to steel, concrete or masonry. Brackets for curtainwalling are commonly fixed to concrete floor slabs but can be fixed to the structural steelframe. Window frames are commonly fixed to masonry but may be fixed to concrete.

SteelFixings to steelwork are normally bolts which may connect directly to structural steel sectionsor to cleats welded to the sections.

Any welding should normally be carried out by the steelwork fabricator prior to delivery tosite.

ConcreteFixings to concrete may be cast-in place or post installed. Cast-in place fixings arepositioned in the formwork prior to casting the concrete and usually take the form ofchannels with T-head bolts or internally threaded sockets, Figure 8.5. There are three formsof post-installed fixings related to their method of load transfer as follows:

- Expansion anchors in which a metal cone is drawn into a metal sleeve or shield causingfriction against the sides of the hole, Figure 8.6. In torque controlled fixings theexpansion occurs as the fixing is tightened. In displacement controlled fixings the sleeveis forced over the cone using a hammer and a separate operation is required to connectthe fixture to the installed fixing.

- Undercut anchors in which the end of the hole is enlarged allowing the end of the anchorto expand without inducing stress in the substrate, Figure 8.7. Mechanical interlock thenprovides resistance to pullout.

- Bonded anchors in which the anchor is held in the hole by resin which may either beintroduced in the form of a glass capsule or may be injected from a cartridge, Figure 8.8.

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Resin anchors transfer the load over the whole of the bonded length giving lower contactstresses than other types of fixing.

The performance of fixings in concrete depends on the strength of the concrete and densityof aggregate. The choice of appropriate fixings will also take account of the practicalproblems of either securing the fixings to the formwork or alternatively drilling holes in thehardened concrete.

MasonryMasonry can be a difficult material to fix into due to the wide range of strength of masonrymaterials, the presence of voids within the masonry units and the presence of mortar joints.Fixings should normally be located within the masonry unit rather than the mortar joint.Fixings for use in masonry include expansion anchors, bonded anchors, screws andspecialist fixings designed for use in low strength materials, particularly aerated concrete

Some expansion anchors with metal sleeves and cones are suitable for use in masonry butsimilar anchors with plastic sleeves and plastic wall plugs are also available. These may bestandard wall plugs were the plug is embedded fully within the masonry and expands when aconventional screw is inserted, Figure 8.9, or frame fixings where the plug extends throughthe fixture into the masonry and may be expanded by a screw or nail, Figure 8.10.

When perforated masonry units are used it may be necessary to use longer fixings which willpass through several webs of material to provide a secure fixing.

Bonded fixings may be used in solid masonry in the same way as they are used in concrete.However where hollow masonry units are used it may be necessary to use a net sleeve tocontain the injected resin, Figure 8.11.

Screws which will cut their own thread in predrilled holes in masonry materials are available,Figure 8.12.

Specialist fixings for use in aerated concrete include plastic plugs with fins which arehammered into predrilled holes, Figure 8.13, and anchors which are grouted into anenlarged hole using a cement grout, Figure 8.14. MaterialsBrackets may be manufactured for a particular installation requiring the specifier to select theappropriate material. Brackets may be made of aluminium, steel or stainless steel. In mostcases proprietary fixings will be used and the choice of material depends on what isavailable. Fixings are commonly available in stainless steel or zinc plated and passivatedsteel. Most stainless steel fixings are available in grade 1.4401(316) but some are alsoavailable in other grades. Fixings may also be available in hot dip galvanised steel andunprotected carbon steel.

Aluminium and stainless steel are durable in most conditions but stainless steel is availablein different grades and an appropriate grade should be selected. Carbon steel componentsrequire protection which is commonly provided by galvanising or zinc plating. Galvanisinggives greater protection than zinc plating but is less durable than stainless steel.

Aluminium, zinc-coated steel and stainless steel are generally compatible in situations whichare likely to occur in practice. Although there is an increased risk of corrosion of aluminiumwhen it is in contact with stainless steel the risk depends on the relative areas of thematerials.

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Stainless steel fixings for aluminium components are therefore acceptable whereasaluminium fixings for stainless steel are not.

The specifier should have taken into account the durability of the materials used and thespecified material and finish must not be changed without his agreement.

Installation

General- Before installation all fixings should be checked to ensure that they are of the specified

type, size and material. Fixings must be installed in accordance with the manufacturer’sinstructions.

- Setting out is required before fixings can be installed. Setting out should be related tothe site datum rather than local features such as the slab edge or nearby column.

- The correct equipment is required. Some fixings require special tools supplied by thefixing manufacturer and may not operate correctly it alternative tools are used.

Cast in fixings in concrete- Fixings should be securely fixed in place before placing the concrete- The concrete should be allowed to cure before applying load to the fixings

Post drilled fixings- Drill hole to correct diameter. Drills become worn with use and need to be replaced at

intervals. Percussive drilling is normally required for concrete but when drilling into weakmaterials rotary drilling may be required to prevent enlargement of the hole.

- In all cases the hole must be deep enough to allow the fixing to be inserted to its fulldepth.

For some fixings a greater depth of hole will not affect the fixing performance. However, for sometypes of fixing, for example bonded fixings with resin capsules and some displacement controlledexpansion anchors, an overlong fixing hole may prevent the correct operation of the fixing.

- Ensure holes are square to the surface.

- Ensure minimum edge distance and spacing is provided. Reducing the edge distanceand spacing reduces the strength of the fixing.

- Ensure reinforcing steel is avoided and agree procedures to be adopted where holesconflict with reinforcement. Reinforcement should only be cut with the agreement of thestructural engineer and when the cut reinforcement will not affect the operation of thefixing.

- Where holes are aborted, due to hitting reinforcement or for any other reason,procedures for filling aborted holes and minimum spacing for replacement holes must beagreed.

- Clean hole thoroughly; blowing is usually sufficient for mechanical anchors, brushing isrequired for bonded anchors.

- For bonded fixings ensure temperature and moisture conditions are suitable and allowresin to cure before applying load.

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- Position fixing correctly.

- Tighten to specified torque using calibrated torque spanner.

If too low a torque is used the anchor may not clamp the fixture securely when subject totensile load and expansion anchors may not give the required pullout strength. Too higha torque may damage the fixing material or may break the bond of resin anchors.

- Fixings should be marked for example by spraying with paint to indicate that the correcttorque has been applied.

Packing and shims- Shims should be made of material with suitable strength and durability. Plastic shims

may be used when fixing window frames but metal shims should be used when fixingbrackets. When metal shims are used the metal must be of sufficient inherent durabilityfor the exposure conditions and be compatible with other metals with which it may comein contact.

- Shims should be of sufficient size to prevent concentrated loads.

- Use of shims will lead to increased bending stresses in fixings subject to shear load.The maximum thickness of shims should be specified and not exceeded.

Slotted holes- Where slotted holes are used to provide adjustment it is important to use washers which

are sufficiently thick to bridge the slot without deformation.

- Where slotted holes are used to provide adjustment but additional movement is to beprevented during the service life a means of locking the fixing is required. Friction underthe clamping action of the fixing is not sufficient.

- This is usually achieved by the use of serrated surfaces. The pitch of the serrationsmust be selected to give sufficiently fine adjustment.

Testing- In most cases proprietary fixings can be used in situations covered by the manufacturer's

test data however occasionally testing may be required to check the suitability of fixings.This is most likely to occur when fixing to an existing structure and the properties of thesubstrate are unknown.

� To check the quality of installation a proportion of the installed fixings may be tested.The test load must be sufficiently high to give a meaningful test but not so high thatcorrectly installed fixings are damaged. Testing is more likely to be required for curtainwall fixings than for window fixings.

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Figure 8.1 Fixings for windows

Figure 8.2 Support and restraint of curtain walling

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Figure 8.3 Fixing points for PVC-u windows

Figure 8.4 Spacing of fixing points

Figure 8.5 Cast-in place fixings

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Figure 8.6 Expansion anchors

Figure 8.7 Undercut anchors

Figure 8.8 Resin anchor

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Figure 8.9 Wall plugs

Figure 8.10 Frame fixings

Figure 8.11 Resin fixing with net sleeve

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Figure 8.12 Self drilling masonry fixings

Figure 8.13 Plastic plug with fins

Figure 8.14 Grouted in anchor